An MRI apparatus is an imaging apparatus which excites nuclear spin of an object placed in a static magnetic field with an RF (Radio Frequency) pulse having the Larmor frequency and reconstructs an image based on MR (Magnetic Resonance) signals generated due to the excitation. An RF pulse changes the rotational directions of nuclear spins of atoms which are aligned into one direction by the static magnetic field (hereinafter, the tilt angle of the rotational axis of the nuclear spin caused by an RF pulse is referred to as a flip angle). The MR signals are generated when excited nuclear spins of an object recover. Thus, an RF pulse, whose intensity gives an intended flip angle, is transmitted in order to excite nuclear spins of an object in an MRI apparatus. This flip angle is one of parameters having influence on image quality, and necessary RF pulse intensity varies depending on, for example, positional relationship between an object and an RF coil etc.
However, output level of RF pulses fluctuates, because it is influenced by gain fluctuation of an amplifier which amplifies RF pulses.
For example, gain of an RF amplifier is influenced by temperature variation. Therefore, an MRI apparatus includes a feedback control mechanism adjusting its temperature by air cooling or water cooling when the temperature of the MRI apparatus increases or decreases by a predetermined value or rate. However, this feedback control mechanism works when the temperature exceeds a predetermined threshold value. The temperature inside the MRI apparatus therefore fluctuates between an upper limit value and a lower limit value each of which is determined as the predetermined threshold value. Although gain adjustment is performed inside the RF amplifier based on difference between its output level and input level, this control is also influenced in synchronization with the temperature fluctuation of the RF amplifier. Thus, RF pulse intensity, which is the final output of the RF amplifier, fluctuates depending on its temperature.
As mentioned above, RF pulse intensity fluctuates moment by moment during implementation term of a scan. In recent years, various imaging protocols are available. When imaging time becomes long, instability of RF pulse intensity due to the above-mentioned temperature variation influences image quality and becomes a factor of artifact.
For the above reasons, an MRI apparatus capable of outputting an RF pulse with intended intensity has been desired.